Process for clarification of an impure acidic titanium sulphate liquor and/or the manufacture of titanium dioxide therefrom

ABSTRACT

The present invention relates to the manufacture of titanium dioxide wherein titanium sulphate liquor is produced containing colloidal and suspended impurities which are flocculated and removed from the liquor before the titanium dioxide is obtained by hydrolysis. More particularly it relates to the improvement of such manufacture comprising mixing the liquor, prior to hydrolysis, with a solution of (a) a polymer of an acrylic acid ester of the general formula: CH2=CR3COO(CH2)xNR1R2 wherein x is 2, 3 or 4, R1 and R2 are each selected from the group consisting of hydrogen and alkyl groups containing up to eight carbon atoms and R3 is a member selected from the group consisting of hydrogen and methyl, or (b) a water-soluble salt of said ester or (c) a copolymer of said ester with an ethylenically unsaturated comonomer, or (d) a copolymer of a water-soluble salt of said ester with an ethylenically unsaturated comonomer; the polymer or copolymer having a molecular weight such that the viscosity of a 1 percent by weight aqueous solution thereof as measured in a No. 3 Suspended Level Viscometer at 25* C. is at least 40 centistokes, the comonomer being either water-soluble and present to the extent of not more than 40 percent by weight of the copolymer or water-insoluble and present to the extent of not more than 15 percent by weight of the copolymer.

United States Patent Rothwell [541 PROCESS FOR CLARIFICATION OF AN IMPURE ACIDIC TITANIUM SULPI-IATE LIQUOR AND/OR THE MANUFACTURE OF TITANIUM DIOXIDE THEREF ROM [72] lnventor: Eric Rothwell, Bradford, England [73] Assignee: Allied Colloids Manufacturing Company Limited, Bradford, Yorkshire, England [22] Filed: July 16, 1970 [21] Appl. No.: 55,587

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 719,710, Apr. 8,

1968, abandoned.

[30] Foreign Application Priority Data Apr. 14, 1967 Great Britain ,.17,l53/67 [52] U.S. Cl. ..23/202 R, 23/117, 209/5, 210/54 [51] lnt.Cl ..Clg 23/04 [58] Field ofSearch ..23/202 R, 117; 75/1 2; 209/; 210/54 [56] References Cited UNITED STATES PATENTS 3,416,885 12/1968 Honchan ..23/202 X 3,069,235 12/1962 Schechter et a1.. .23/202 3,023,162 2/1962 Fordyce et al.. ..75/] X 3,418,237 12/1968 Booth et al..... .....75/l X 2,909,508 /1959 Jones ..75/2 X [451 Apr. 25, 1972 FOREIGN PATENTS OR APPLICATIONS 776,503 6/1957 Great Britain Primary Examiner- Herbert T. Carter Attorney- Beveridge & Degrandi [57] ABSTRACT The present invention relates to the manufacture of titanium dioxide wherein titanium sulphate liquor is produced containing colloidal and suspended impurities which are flocculated and removed from the liquor before the titanium dioxide is obtained by hydrolysis. More particularly it relates to the improvement of such manufacture comprising mixing the liquor, prior to hydrolysis, with a solution of (a) a polymer of an acrylic acid ester ofthe general formula:

wherein an is 2, 3 or 4, R and R'- are each selected from the group consisting of hydrogen and alkyl'groups containing up to eight carbon atoms and R is a member selected from the group consisting 'of hydrogen and methyl, or (b) a watersoluble salt of said ester or (c) a copolymer of said ester with an ethylenically unsaturated comonomer, or (d) a copolymer of a water-soluble salt of said ester with an ethylenically unsaturated comonomer; the polymer or copolymer having a molecular weight such that the viscosity of a 1 percent by weight aqueous solution thereof as measured in a No. 3 Suspended Level viscometer at C. is at least centistokes, the comonomerbeing either water-soluble and present to the extent of not more than 40 percent by weight of the copolymer or water-insoluble and present to the extent of not more than 15 percent by weight of the copolymer.

8 Claims, No Drawings PROCESS FOR CLARIFICATION OF AN IMPURE ACIDIC TITANIUM SULPHATE LIQUOR AND/OR THE MANUFACTURE OF TITANIUM DIOXIDE THEREFROM This application is a continuation-in-part of Application Ser. No. 719,710 filed Apr. 8, 1968, and now abandoned.

The present invention concerns improvements in clarification, particularly of titanium sulphate liquor and the sedimentation of the impurities contained therein.

The early stages in the manufacture of titanium dioxide pigment by the so-called Sulphate Process comprise the sulphation of titaniferous ores or slags with sulphuric acid to produce eventually an aqueous acidic liquor containing the sulphates of titanium and other metals in solution. Such liquors contain also suspended insoluble impurities in particulate or colloidal form which are largely comprised of undissolved ore and siliceous or earthy matter.

Ultimately the titanium dioxide is obtained by hydrolysis of the purified acidic liquor and at some time prior to the hydrolysis it is necessary to free the liquors from the above mentioned insoluble impurities. This is commonly achieved by a clarification stage in sedimentation tanks, and the clarification process may be aided by a subsequent filtration stage.

It is common practice to assist clarification by the introduction of a clarification reagent and to this end, in many cases, a compound of antimony is introduced at the sulphation stage so that antimony is present in a soluble form in the titanium sulphate liquor.

On treating the liquor subsequently, with sodium sulphide a flocculant precipitate of antimony sulphide is formed, which in the course of its formation and settling, coagulates and causes co-settling of the above mentioned impurities in a convenient manner.

This use of antimony as a clarification aid suffers however from several disadvantages. These include the not insubstantial cost of treatment, limitations in the speed of clarification which is obtainable, and the liberation of the toxic and corrosive hydrogen sulphide gas during precipitation.

There have been several attempts in the past to achieve this clarification by means of organic reagents such as, for example, surface-active agents of the sulpho-succinate type and particularly polymeric agents of the hydrolyzed protein class. However, the low efficiency of such materials has precluded or restricted their use on a large scale.

More interest has recently been attached to the use of synthetic polymeric electrolytes which have a flocculating and sedimenting action because of their ability to be adsorbed onto the surface of suspended particles by virtue of polar groups on the polymer chain, and which further because of their molecular size have the ability to form bridges between particles, thus causing them to cluster into large aggregates having superior settling characteristics to the individual particles.

One particular group of synthetic polymers which has been proven to be economically effective as a means of clarification of titanium sulphate liquors comprises the so-called Polyamide-epichlorhydrin resins such as have found employment in the paper industry because of their flocculating efficiency towards paper constituents. Such products are based on resins formed by the condensation of polyamines, e.g. diethylene triamine and dibasic acids, e.g. adipic acid. Although such products have proven to be ofv value as clarification aids for titanium sulphate liquor, they suffer from certain limitations. Thus the polymer chain contains amide groups which are susceptible to hydrolysis with consequent reduction of the polymer chain length so that on storage, hydrolytic degradation reduces the effectiveness of the products. At the operating temperature 50 C. or higher of the clarification process the tendency to degradation is aggravated and in many cases severely limits both the clarification action and the subsequent settling characteristics of the flocculated impurities. Furthermore, such products do not appear to be capable of preparation at very high molecular weights, which limits their efficiency. Attempts to produce higher molecular weights by reaction with cross-linking reagents, e. g. epichlorhydrin, limits the stawhen stored at commercially useful concentrations.

Many other synthetic polyelectrolytes are mentioned in the literature as being useful as flocculating agents for mineral suspensions. It is in fact true to state that all polyelectrolytes can produce flocculation of suspended particles in aqueous solution when the polyelectrolyte carries the appropriate polar groups and has a large molecular size when in solution. In this respect the molecular weight need not be very high for a flocculating or aggregating effect to be detectable visually on addition of the polymer to a particular suspnesion of particles. The molecular weight can, in fact, be as low as say 20,000.

Thus, British Patent No. 768,665 describes the use of synthetic cationic polyelectrolytes as aggregating agents and mentions particularly molecular weights of 15,000 to 100,000. Included in the examples of active compositions are polymers based on acrylamide, polymers derived from cationic acrylates such as dimethyl amino ethylmethacrylate and various others for example polymers derived from vinyl pyridine. Likewise British Patent No. 776,503 suggests the use of polymers derived from cationic acrylates, specifically N-tertiary alkyl amino alkyl acrylates, for the purpose of agglomerating soil.

However, in the present day application of polyelectrolyte fiocculants as industrial sedimentation aids as compared with their use as soil conditioning or filtration aids there are many important factors which govern the choice of the flocculating agent. The principal ones are on the one hand the nature of the substrate being treated and the equipment in which the treatment is to be carried out and on the other hand the character of the flocculation to be produced. With respect to the latter property for instance the major effects required may be clarity of supernatant liquor, or the initial rate of settlement, or the overall rate of settlement, or the degree or rate of compaction of the settled aggregates or the redispersion characteristics of the settled aggregates or combinations of two or more such properties. With all these requirements to fulfill therefore a selection of polymer characteristics is necessary and these depend in a highly specific manner .on the chemical structure of the polymer and its molecular weight, and its physical structure in solution.

It has been found for instance that the cationic polyelectrolytes as quoted in the above mentioned patents do not directly yield products which are effective fiocculants for the clarification of titanium sulphate liquor. Thus vinyl pyridine polymers have found to be inactive. Cationic acrylate polymers are likewise inactive unless their composition and molecular weights fall within the ranges of the present invention. Polyacrylamide is inactive. Co'polymers of acrylamide and a cationic acrylate, more specifically a quaternary salt derived from the methylation of an alkyl amino alkyl acrylate, wherein the copolymers contain at the most 40'percent of the cationic constituent and which have a molecular weight exceeding 1 million and which are extremely effective flocculants and sedimentation aids in the paper and various mineral processing industries are likewise inactive in the clarification of titanium sulphate liquor.

ln recent years another major class of synthetic polyelectrolyte fiocculants has become important as industrial clarification and sedimentation aids. These comprise the anionic polyacrylamides and while British Patent No. 768,662 covers the use of anionic polyelectrolytes generally as aggregating agents, more recently the outstanding advantage of specificity with regard to molecular strucutre for industrial sedimentation work has become apparent for example in British Patent No. 901,916 where high molecular weight copolymers of acrylic acid and acrylamide are cited, the acrylamide content of such copolymers being specifically present within the proportions of 75 to 45 percent. Here again it has been found that such polymers are of little effect as clarification aids for titanium sulphate liquor.

weights ranging from about 2,000 to 100,000 and because of their relatively high charge density, namely one positively charged nitrogen atom per three atoms of the polymer chain, are considered to have exceptional ability for the coagulation and flocculation of fine particle suspensions and consequently find employment in the paper-making and sewage treatment fields. These products likewise have been found to be ineffective for the clarification and sedimentation of titanium sulphate liquor.

It is a fact, therefore, that a very wide range of synthetic polyelectrolytes which, because of their ionic charge, are capable of acting as aggregating agents or which, because of their high molecular weight and/or their chemical composition, are used as economical and effective sedimentation and clarification aids, show surprisingly little or no effect on the clarification of titanium sulphate liquors.

It appears, therefore, that the clarification of such liquors demands of the flocculant some important properties additional to the normal parameters such as molecular weight, charge type and charge density which govern most flocculant applications. It is particularly surprising to find that the required properties are shown by the narrow range of synthetic cationic polymers and copolymers whose usage in titanium sulphate clarification is the object of this invention, and that such materials also show surprisingly high resistance to degradation in that they maintain their effectivenesss as flocculants and sedimentation aids throughout many hours of exposure to the hot, strongly acidic environment of the titanium sulphate settling tanks and thereafter in subsequent processes.

According to the present invention therefore in a process for the manufacture of titanium dioxide wherein titanium sulphate liquor is produced containing colloidal and suspended impurities which are flocculated and removed from the liquor before the titanium dioxide is obtained by hydrolysis, the improvement which comprises mixing the liquor, prior to hydrolysis, with a solution of (a) a polymer of an acrylic acid ester of the general formula:

wherein .r is 2, 3 or 4, R and R are each selected from the group consisting of hydrogen and alkyl groups containing up to eight carbon atoms and R is a member selected from the group consisting of hydrogen and methyl, or (b) a water-soluble salt of said ester or (c) a copolymer of said ester with an ethylenically unsaturated comonomer, or (d) a copolymer ofa water-soluble salt of said ester with an ethylenically unsaturated comonomer; the polymer or copolymer having a molecular weight such that the viscosity of a 1 percent by weight aqueous solution thereof as measured in a No. 3 Suspended Level Viscometer at 25 C. is at least 40 centistokes, the comonomer being either water-soluble and present to the extent of not more than 40 percent by weight of the copolymer or water-insoluble and present to the extent of not more than percent by weight ofthe copolymer.

All the viscosity measurements referred to in this specification were measured at 25 C. in a suspended level viscometer Type Mark BS/IP/SL as described in British Standard Specification 188, 1957.

It is preferred to use a polymer or copolymer whose l percent by weight aqueous solution viscosity as measured in a Number 3 suspended level viscometer at 25 C. lies within the range 80 to 200 centistokes.

If a polymer or copolymer having too high a molecular weight, is used this can cause difficulties in handling and in distribution in the liquor.

it is preferred to use copolymers of the cationic esters of acrylic or methacrylic acid as defined above or of a simple salt or quaternary salt thereof with an ethylenically unsaturated monomer. If this comonomer is water-soluble it is present to the extent of not more than 40 percent by weight, more particularly from 3 to 30 percent by weight, of the copolymer. If, on the other hand, it is a water-insoluble comonomer then it is present to the extent of no more than 15 percent by weight, more particularly 3 to 10 percent by weight, of the copolymer.

Preferred water-soluble comonomers are vinyl pyrrolidone, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, the mono-alkyl esters of maleic acid or fumaric acid; the amides or their hydroxyethyl or hydroxypropyl esters of any of these acids and alkyl vinyl ethers.

Preferred water-insoluble comonomers are the alkyl esters derived from acrylic or methacrylic acid and alcohols of general formula c li Ol-l where n l to 8, styrene, methyl styrene, vinyl acetate, vinyl propionate, vinyl butyrate, and acrylonitrile.

The polymers and copolymers of this invention can be prepared for example by vinyl polymerization of mixtures of the comonomers with redox initiator systems, or in some cases by chemical modification of a pre-formed polymer. Thus for example a copolymer of methacrylic acid and a dialkyl amino methacrylate may be prepared by polymerizing in aqueous solution mixture of the two monomers using a persulphate as an initiator or alternatively by chemical modification such as by hydrolysis of an ester homopolymer e.g. by controlled hydrolysis of polydimethyl amino methacrylate.

The invention will be more clearly understood by reference to the following examples. In all these examples the products being tested are introduced into the titanium sulphate liquor in the form of their 0.5 percent by weight aqueous solution. The dosage level is indicated as parts per million on the liquor meaning parts by weight of polymer or copolymer on the liquor quantity EXAMPLE 1 Titanium sulphate liquor was prepared by dissolving 1,500 gm. of sulphated ilmenite ore in 1,500 ml. of water at 65 C. This solution was reduced by treatment with iron filings until a diluted sample of the liquor no longer gave a red coloration with ammonium thiocyanate. The reduced liquor was finally decanted from undissolved iron and the density adjusted to 1.6 by the addition of water. Liquor prepared in this manner is typical of that occurring during the industrial preparation of titanium dioxide pigment from ilmenite by the sulphate process.

To 50 ml. quantities of this liquor, the products of the invention and products of known flocculating activity were added at a concentration of parts per million and with thorough mixing. The treated liquor was allowed to stand at 65 C. and examined for clarification effects.

The results are given in Table 1.

By strong flocculation is meant a clearly formed floc structure which settles fairly rapidly so that there is less than 50 percent settled solids volume in 10 minutes.

By moderate flocculation is meant a clearly formed floc structure settling slowly so that there is less than 50 percent settled solids volume in 40 minutes.

By slight flocculation is meant a floc structure is evident but there is little tendency to settle so that after standing for 1 hour a well defined settled solids/liquor interface is not apparent.

It can be seen from these results that a very wide variety of commercial flocculant types have been examined but that only two narrow groups of compounds are effective. These are the polyamide-epichlorhydrin resins and products of this invention as described in detail earlier.

TABLE 1 Flocculating agent added Result Acetate of copolymer of diethyl amino ethyl acrylate and cetyl methacrylate in weight ratio 50:50. 1% solution viscosity 3.4 centistokes.

. Copolymer of methylated diethylamino ethyl acrylate and acrylamide in weight ratio 40:60. 1% solution viscosity 2.6 centistokes.

. Polymer of methylated diethylamino ethyl acrylate. 1% solution viscosity 4.3 centistokes.

. Copolymer of methylated diethylamino ethyl acrylate and acrylamide in weight ratio 60:40. 1% solution viscosity 195 centistokes Acetate of polymer of diethylamino ethyl acrylate. 1% solution viscosity 43 centistokes.

6. Polymer of methylated diethylamino ethyl acrylate, 1% solution viscosity 56 centistokes.

. Copolymer of methylated diethylamino ethyl acrylate and acrylamide in weight ratio 70:30. 1% solution viscosity 150 centistokes Copolymer of sodium acrylate and acrylamide in weight ratio :90. 1% solution viscosity 230 No action Slight flocculation Slight flocculation Moderate flocculation LII Strong flocculation Strong flocculation Strong flocculation centistokes. Slight flocculation 9. Copolymer of sodium acrylate and acrylamide in weight ratio 40:60. 1% solution viscosity 6,300 centistokes. Slight flocculation l0. Polystyrene sulphonic acid, 5%

solution viscosity 26 centistokes. No Action 1 1. Sodium dioctyl sulphosuccinate. No Action 12. Polyacrylamide, 1% solution viscosity'400 centistokes. Slight flocculation 13. Polyvinyl pyrrolidone. 30%

solution viscosity 180 centistokes. Slight flocculation 14. Norwegian animal glue. Slight flocculation l5. Triethylene tetramine No Action 16. Polyethylene-imine.

solution viscosity 950 centistokes Slight flocculation l7. Polyamide epichlorhydrin resin. 10% solution viscosity 1200 centistokes.

18. Cationic condensation product of tannin, ethanol-amine and formaldehyde according to Strong flocculation British Pat. No. 899,721. No Action 19. Hydrosulphate ofpolyvinyl pyridine. as described in British Pat. No. 768,665. Slight flocculation EXAMPLE 2 A titanium sulphate liquor was prepared according to the procedure defined in Example 1.

The polymers used in this example were copolymers prepared by free radical polymerization of mixtures of diethyl amino ethyl acrylate quaternised by dimethyl sulphate and hydroxy ethyl or hydroxy propyl acrylate. The products were tested by adding them at a concentration of 100 parts per million to 250 ml. quantities of titanium sulphate liquor held at a temperature of 65 C. in an open beaker. The liquor was stirred for seconds at 300 revolutions per minute after the addition of the reagents and thereafter for 30 seconds at 150 revolutions per minute. The beaker contents were then transferred to a measuring cylinder and the rate of settlement of suspended solids observed.

The results are given in Table 2.

In this test an efficient reagent will produce a settled solids volume comprising 30 percent of the 'total liquor volume within 30 minutes. A higher settled solids volume after this time would be an unfavorable result in plant practice.

TABLE 2 Polymer composition 1% solution volume of viscosity settled solids (centiafter 30 minstokes utes 5 parts of hydroxy ethyl acrylate (HEA) 95 parts of quaternised diethyl amino ethyl acrylate (q DEAEA) 35 65 do- 88 28 do- 156 25 10 parts of HEA 90 parts ofq DEAEA 2O 53 do 53 26 do-- 125 21 20 parts of HEA 80 parts of q DEAEA 38 60 do 195 27 30 parts of HEA 70 parts ofq DEAEA 32 67 do- 29 50 parts of I-IEA 50 parts ofq DEAEA 430 No effect 10 parts of hydroxy propyl acrylate (HPA) 32 48 90 parts of quaternised diethylamino ethyl acrylate (q DEAEA) -do- 69 28 20 parts of HPA 80 parts ofq DEAEA 35 50 do- 50 30 -do 350 24 40 arts of EPA 60 parts ofq DEAEA 78 30 do 540 28 50 parts of HPA 50 parts ofq DEAEA 870 No effect EXAMPLE 3 To ml. quantities of titanium sulphate liquor, prepared as in Example 1 above and held in measuring cylinders at 65 C. were added various amino alkyl acrylates as solutions in dilute acetic acid. The dosage of reagent was 75 parts per million on the volume of liquor used. Immediately after addition of the reagents the cylinder contents were shaken by hand to disperse the reagent. The volume of sediment accumulating after 20 minutes standing was then measured.

In this test a sediment volume at 20 minutes amounting to 35 percent of the total liquor volume would be an acceptable result in plant practice.

A titanium sulphate liquor was prepared according to the procedure outlined in Example 1. To 50 ml. quantities of the liquor in measuring cylinders were added parts per million of various reagents which were mixed by thorough shaking. Q After allowing to stand in a water bath at 65 C. for 15 minutes gigs/meth l '41 N the degree of settlement and clarification of the supernatant go zo 90 liquor were assessed visually. 85/15 77 30 in this test a satisfactory result would be a settled solids 95/5 104 28 volume amounting to less than 40 percent of the total liquor gg g 53 70 volume and a supernatant liquor visibly free from suspended 85/15 H 62 30 particles. 80/20 Acetate of TABLE 4 Percent volume of settled solids Clarity 01 after supernatant Polymer composition Solution viscosity minutes liquor Poiyamidc-epichiorhydrin 1i centistokes at 1 35 Fairly clear.

210 centistokes at 1%. 47 Very cloudy. Polyethylene imine 4 centistokes at Very little efiect. 60/40 Acrylamide/sodium acrylate copolymer 4,000 eentistokes at 1% 00/10 Acryiamide/quaternised-diethylamino ethyl 140 centlstokes at 1%....

ucrylute (q. DEAEA). Acrylamide/q. DEAEA:

60/40 195 centistokes at 1%.... 58 Very cloudy.

/50 213 centistokes at 1%... 55 D0. /70 670 centistokes at 1%.... 39 Slightly cloudy. 20/80 170 centistokes at 1%.. 34 Clear. 10/00 75 centistokes at 1%.. 30 Do. 5/95 64 centistokes at 1%..... 32 Do.

I No action.

EXAMPLE 5 DEAEA styrene 47 No action 85/15 50 Following the procedure of Example 3 the following results 30 80/20 Acetate of were obtained using polymers from dimethylsuiphate quaterafifykmlmle nized diethyi amino ethyl acrylate with various other 5 93 32 monomers. The dosage of reagent used was 200 parts per million on the liquor treated.

35 EXAMPLE 6 TABLE 5 One thousand milliliters quantities of freshly prepared titanium sulphate liquor were gently agitated in beakers Polymer 1% Solution volume of placed in a water bath maintained at 65 C. The products to be composition viscosity settled Solids tested were added at a dose rate of 100 parts per million based (centistokes) after 20 on the liquor quantity, and agitation continued for a period of mes 3 hours. At various intervals of time, 250 ml. samples were withdrawn from the treated liquors and the settlement charac- /50 Q teristics assessed visually in measuring cylinders by first invert- DEAE ry i 45 ing the stoppered cylinders four times to homogenize the con- Acid 53 No action /30 68 34 tents and then measuring the rate of solids settlement. 80/20 0 83 33 The products compared and the results obtained are quoted 90/10 62 27 in Table 6.

TABLE 6 Heq'ting Settled solids volume at (minutes) [1118 Polymer composition (hours) 4 6 8 10 12 20 A. P0iyamidc-epiciilorhydrin resin. 11 centistokes at 10% g g 8 g 8 3 ii. Qnnternisod dicthylainino ethyl acrylate. 33 centistokes at 1% 8 g g :23 C. Quntorniscd diethyimnino ethyl ncrylate. centistokes at 1% 1 3 8 g 38 1). 10/00 Methyl ncrylnto/Q DEAEA. 72 centistokes at 1% g {g2 E. 10 00 Acrylamide/Q DEAEA. 75 ccntistokes at 1% g 8 38 2g f.

4 /6 0 DEAEA From these results it can be seen that product A has lost za' fifi g; No 65 nearly all ability to flocculate after only 1% hours, due to its 70 28 destruction by hydrolysis. The other products are seen to be 40/60 0 DEAEA/N resistant to hydroiytic degradation, their floc holding effects methyl airylamide 620 NO aclio" differing mainly in the resistance to the disruptive action of 60/40 H 850 the continuous agitation. 80/20 170 30 40/60 Q DEAEA/vinfl 70 Another advantage conferred by the use of the products of pyrrolidone 128 No action the invention lies in a subsidiary process commonly carried 70/38 100 17 out during the manufacture of titanium dioxide by the sulphag i iso tion process, and which involves washing of the settled solids bury] maleam 96 5 from the titanium sulphate clarifier in a further solids-settling 70/30 33 75 system, in order to recover the valuable titanium-bearing liquor entrained therein. Because of the nature of the process the resedimentation characteristics of the washed solid particles are once more of importance and it is common practice to use sedimentation aids at this stage also in order to achieve rapid separation of the solids from the wash liquors. When however the titanium sulphate liquoris clarified using the roducts of the invention it has been found that the strong floc structure thereby produced persists to a considerable extent throughout the water-washing treatment and facilitates the final solids-liquid separation thus reducing or eliminating the need for additional fiocculating aids.

EXAMPLE 7 In this example, the settled solids from the 2 hour agitation test of the type described in Example 6 were isolated by decanting off the supernatant liquor. The settled solids were transferred to 250 ml. graduated cylinders and after the addition of 200 ml. of water, were homogenized by inverting four times. The settling characteristics were then noted after 3 minutes standing.

The results obtained were as follows:

Product A gave a settled solids volume of 68 ml. and extremely cloudy supernatant liquor.

Product B gave a settled solids volume of 48 ml. and cloudy supernatant liquor.

Product E gave a settled solids volume of 32 ml. and clear supernatant liquor.

I claim:

1. In a process for the manufacture of titanium dioxide wherein titanium sulphate liquor is produced containing colloidal and suspended impurities which are flocculated and removed from the liquor before the titanium dioxide is obtained by hydrolysis, the improvement which comprises mixing the liquor, prior to hydrolysis, with a fiocculating solution of (a) a copolymer of an acrylic acid ester of the general formula:

Cl-l CRCOO (Cl-l NRR, wherein x is 2, 3 or 4, R and R are each selected from the group consisting of hydrogen and alkyl groups containing up to eight carbon atoms and R is a member selected from the group consisting of hydrogen and methyl, with an ethylenically unsaturated comonomer, or (b) a copolymer of a watersoluble salt of said ester with an ethylenically unsaturated comonomer; the copolymer having a molecular weight such that the viscosity of a 1 percent by weight aqueous solution thereof as measured in a No. 3 Suspended Level viscometer at 25 Co. is at least 40 centistokes, the comonomer being either water-soluble and present to the extent of not more than 40 percent by weight of the copolymer or water-insoluble and present to the extent of not more than 15 percent by weight of the copolymer whereby said colloidal and suspended impurities are flocculated and-thereafter removing said impurities from the liquor.

2. Process according to claim 1 in which the copolymer is one whose 1 percent by weight aqueous solution viscosity measured in a No. 3 suspended level viscometer at 25 C. lies within the range to 200 centistokes.

3. Process according to claim 1 in which the copolymer is derived from N ,N-diethylamine ethyl acrylate or a water-soluble salt thereof.

4. Process according to claim 1 in which the comonomer is water-soluble and present to the extent of 3 to 30 percent by weight of the copolymer.

5. Process according to claim 1 in which the comonomer is acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, a water-soluble mono-alkyl ester of maleic or fumaric acid, an amide or hydroxyethyl ester or hydroxypropyl ester of any of these acids, vinyl pyrolidone or a water-soluble alkyl vinyl ether.

6. Process according to claim 5 in which the amide is acrylamide or methacrylamide.

7. Process according to claim 1 in which the comonomer is water-insoluble and present to the extent of from 3 to 10 percent by weight of the copolymer.

8. Process according to claim 1 in which the comonomer is water-insoluble and is an alkyl ester derived from acrylic or methacrylic acid and an alcohol of the formula C,,H OH

where n is l to 8, styrene, methyl styrene, vinyl acetate, vinyl V propionate, vinyl butyrate or acrylonitrile. 

2. Process according to claim 1 in which the copolymer is one whose 1 percent by weight aqueous solution viscosity measured in a No. 3 suspended level viscometer at 25* C. lies within the range 80 to 200 centistokes.
 3. Process according to claim 1 in which the copolymer is derived from N,N-diethylamine ethyl acrylate or a water-soluble salt thereof.
 4. Process according to claim 1 in which the comonomer is water-soluble and present to the extent of 3 to 30 percent by weight of the copolymer.
 5. Process according to claim 1 in which the comonomer is acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, a water-soluble mono-alkyl ester of maleic or fumaric acid, an amide or hydroxyethyl ester or hydroxypropyl ester of any of these acids, vinyl pyrolidone or a water-soluble alkyl vinyl ether.
 6. Process according to claim 5 in which the amide is acrylamide or methacrylamide.
 7. Process according to claim 1 in which the comonomer is water-insoluble and present to the extent of from 3 to 10 percent by weight of the copolymer.
 8. Process according to claim 1 in which the comonomer is water-insoluble and is an alkyl ester derived from acrylic or methacrylic acid and an alcohol of the formula CnH2n 1OH where n is 1 to 8, styrene, methyl styrene, vinyl acetate, vinyl propionate, vinyl butyrate or acrylonitrile. 